Can't install software in Ubuntu Software Centre

Is any other package download app open? Like Synaptic and Update manager. If so, close them and you should be able to install.

If that doesn't solve your problem, please follow this procedure and post the results (you can post them here, you needn't start a new question for them):
https://help.ubuntu.com/community/PackageManagerTroubleshootingProcedure

I noticed the same error a few times now too, 11.04.

@Joshua
Did following the instructions at https://help.ubuntu.com/community/PackageManagerTroubleshootingProcedure solve your problem? If so, you can mark this question as Solved (you can do that here at https://answers.launchpad.net/ubuntu/+source/software-center/+question/143661). If not, please reply with the information you obtained from following that procedure, including all the text from the Terminal.

@David
Please only use the "Add Answer" button if you are proposing a solution to the asker's problem, or if the question's status is not Answered but you are confident that it has been answered (for example, if someone else just provided an answer, but clicked a different button, making it an information request or comment). To provide information that you think might be useful, you can use the "Just Add a Comment" button. If you are still experiencing this specific problem, I recommend you perform the steps at https://help.ubuntu.com/community/PackageManagerTroubleshootingProcedure, and if that does not solve the problem, create a new question (https://answers.launchpad.net/ubuntu/+source/software-center/+addquestion) about your problem--you can post a link to the new question here, and include a link in the new question in this question by leaving a comment.